CCD type solid state imaging device

Abstract: A CCD type solid state imaging device including a plurality of vertically arranged light receiving elements, a plurality of vertical charge transfer electrodes associated with the light receiving elements and transfer channels through which signal electrical charges stored in the light receiving elements are readout and transferred in the charge transfer direction by the transfer electrodes. The present invention features that unlike the conventional CCD type solid state imaging device, the boundary line between at least a part of the vertical charge transfer electrodes and a part of the vertical charge transfer electrodes adjacent thereto is held aslant with respect to the transfer channels so that the vertical transfer efficiency of the device is increased without incurring a reduction in the amount of signal charges being handled and the readout of the signal charges stored in the light receiving elements to the transfer channels is performed more smoothly than the conventional device.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a CCD type solid state imaging device and more particularly, to a novel CCD type solid state imaging device which is capable of improving the vertical charge transfer efficiency and the efficiency of reading out signal charges from the light receiving elements to the transfer channels. 2. Description of the Related Art The conventional CCD type solid state imaging device has vertical charge transfer electrodes arranged such that the orientation of the boundary lines among multilayer vertical charge transfer electrodes lying over the transfer channels is normal to the charge transfer direction. In other words, the vertical transfer electrodes of the CCD imaging device represent, as a whole, a shape which would be formed by a plurality of rectangles if they were shifted parallelly so as to partially overlap one another with the horizontal sides of each of the vertical charge electrodes running normal to the transfer direction as will be described in detail later. However, in view of the fact that it is considered necessary for a CCD type solid state imaging device to have a higher charge transfer efficiency, an attempt has been made to improve the charge transfer efficiency in such a manner that a shallow barrier of an oblique pattern is formed by selectively implanting impurities in the transfer channels thereby meeting such demand but this method is not desirable since it reduces the amount of electrical charges being handled, in proportion to the area of the barrier. Prior art document PATENT ABSTRACTS OF JAPAN, Vol. 14, No. 414 (E-975), Sept. 1990, & JP-A-2 159 063 discloses a solid-state image-sensing device, wherein the boundary of transfer electrodes is inclined in the direction of transfer so that a region where a potential difference occurs when a voltage is applied to the transfer electrode becomes longer than before. Further, prior art document EP-A-0 097 837 describes a charge transfer device, wherein a plurality of electrodes is arranged along a charge transfer direction. Each of these electrodes has a shape in plan view which is intended or concave in the charge transfer direction for producing a potential well in the semiconductor region. SUMMARY OF THE INVENTION It is an object of the invention to provide a CCD type solid-state imaging device which is capable of improving the vertical charge transfer efficiency without reducing the amount of signal charges being handled and which enables the signal readout from the light receiving elements to the transfer channels to be performed smoothly. To solve this object, the present invention provides a CCD type solid-state imaging device as specified in the claim. A first aspect resides in that the boundary line between at least a part of the adjoining vertical transfer electrodes is so oriented that when viewed from above, either the edge of the boundary line intersecting the side of each of the transfer channels on the side of the light receiving element associated with the electrodes or the edge of the boundary line intersecting the side of that transfer channel on the side opposite the light receiving element is biased toward the transfer direction with respect to the other edge. Consequently, the transfer field strength in the case of applying the same voltage on the vertical transfer electrodes becomes larger than that in the case of the conventional device due to the horizontal two-dimensional contour effect and the potential gradient along the transfer direction under the vertical transfer electrodes can be made larger, to thereby improve the vertical charge transfer efficiency. A second aspect resides in that the boundary line between the vertical transfer electrode which reads out a signal charge from the light receiving element associated therewith to the transfer channel related thereto, and the vertical transfer electrode adjacent the former electrode is so oriented that when viewed from above, the edge of the boundary line intersecting the side of the related transfer channel on the side opposite the light receiving element is biased toward the transfer direction with respect to the edge of the boundary line intersecting the side of the transfer channel lying on the side of the light receiving element. Consequently, in addition to the effect of improving the vertical charge transfer efficiency as achieved by the imaging device according to the first aspect, the reading out of signal charges from the light receiving element by the vertical transfer electrode associated with that element to the related transfer channel can be performed smoothly. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A, 1B and 1C show an example of a conventional CCD type solid state imaging device of which Fig. 1A is a plan view of the same, Fig. 1B is a sectional view of an electrode structure of the device when taken along the B-B line of Fig. 1A and Fig. 1C is a potential profile taken along the B-B line of Fig. 1A. Figs. 2A, 2B and 2C show one embodiment of a CCD type solid state imaging device not covered by the present invention but helpful for understanding the present invention of which Fig. 2A is a plan view of the same, Fig. 2B is a sectional view of an electrode structure of the same when taken along the B-B line of Fig. 2A and Fig. 2C is a diagram showing a potential profile taken along the B-B line of Fig. 2A; Fig. 3 is a graph showing relationships between the voltage to be applied upon vertical charge transfer electrodes included in the CCD type solid state imaging device and the field strength under the vertical charge transfer electrodes; Fig. 4 is a plan view of an embodiment of the CCD type solid state imaging device according to the present invention; and Fig. 5 is a diagram showing the structure of an interline CCD type image sensor to which the present invention is applied. PREFERRED EMBODIMENTS OF THE INVENTION Prior to describing a preferred embodiment of the present invention, a conventional CCD type solid state imaging device will be described with reference to Figs. 1A, 1B and 1C. As shown in these figures, a conventional CCD type solid state imaging device includes transfer channels 1, channel stoppers 2, light receiving elements 3, a vertical charge transfer electrode 4 comprising a first polysilicon layer (1 Poly), a vertical charge transfer electrode 5 comprising a second polysilicon layer (2 Poly) and a vertical charge transfer electrode 6 comprising a third polysilicon layer (3 Poly). The electrodes 4, 5 and 6 are basically arranged according to a vertical stripe pattern. As will be understood from Fig. 1A, the orientation of the portions of the boundary line between the multilayer-vertical charge transfer electrodes lying over the transfer channels has been such that it runs normal to the charge transfer direction as already mentioned. That is, the boundary line between the vertical charge transfer electrodes is normal to the transfer direction. Fig. 1B shows an electrode structure in section when taken along B-B line of Fig. 1A. From this figure it will be seen that the potential of the (1 poly) vertical charge transfer electrode 4 is -9 V, the potential of the vertical charge transfer electrode 5 is 0 V and the potential of the (3 poly) vertical charge transfer electrode 6 is -6 V. Fig. 1C shows a potential profile at the time of transferring a signal charge from under the (3 poly) electrode 6 to under the (2 poly) electrode 5 in the potential conditions as shown in Fig. 1B. In this figure, the broken line designates a potential gradient (an angle of ) at the portion below the (3 poly) vertical charge transfer electrode 6 (i.e. the portion where the (1 poly) and (2 poly) vertical charge transfer electrodes 4 and 5 are absent). A CCD type solid state imaging device will now be described in detail with reference to Figs. 2A through 2C. In these figures, reference numeral 21 designates a transfer channel, reference numeral 22 designates a channel stopper, reference numeral 23 designates a light receiving element, reference numeral 24 designates a vertical transfer electrode comprising a first polysilicon layer, reference numeral 25 designates a vertical transfer electrode comprising a second polysilicon layer and reference numeral 26 designates a vertical transfer electrode comprising a third polysilicon layer. These elements are basically arranged according to a vertical stripe pattern. The difference between this CCD type solid state imaging device and the above conventional one resides in that the boundary line of the adjoining vertical transfer electrodes is formed such that when viewed from above, either one of an edge a of the boundary line intersecting the side of the transfer channel 21 on the light receiving element side or an edge b thereof intersecting the side of the transfer channel 21 on the side opposite the light receiving element 23 is biased toward the transfer direction with respect to the other, and in the instant embodiment, the edge b is biased toward the transfer direction with respect to the edge a. Thus, with the above arrangement, the potential profile in the course of transfer of a signal charge from the third layer polysilicon vertical transfer electrode 26 to the second layer polysilicon vertical transfer electrode 25 will be as shown in Fig. 2C wherein the broken line designates the minimum field strength distribution gradient (angle 0) under the vertical transfer electrode 26. This gradient is sharper than that which is obtained with the conventional CCD type solid state imaging device shown in Figs. 1A to 1C. Further, as shown in Fig. 3 and the table I below, the transfer field [m V/µm] when the same voltage (i.e. V3 [V]) is applied upon the third layer vertical transfer electrode 26 is generally stronger than that in the case of the conventional device. By the way, Fig. 3 shows relationships between the gate voltage applied on the third layer vertical transfer electrodes of both the conventional device and the instant embodiment and the transfer field strengths under such vertical transfer electrodes wherein the solid lines designate a plurality of cases according to the instant embodiment and the two-dot chain line designates a case according to the conventional device. Further, the following table I illustrates a number of cases in both the instant embodiment and the conventional device of relationships between the gate voltage applied on the vertical transfer electrodes and the transfer field strengths occurring under the vertical transfer electrodes. Thus, since the edge b of the boundary line between the adjoining vertical transfer electrodes intersecting the side of the transfer channel lying on the side opposite the light receiving element associated with the electrodes is biased toward the transfer direction with respect to the edge a of the boundary line intersecting the side of the transfer channel facing the light receiving element, the direction in which the signal charges are read out from the light receiving element to the transfer channel becomes aslant when viewed from above, thereby smoothing the charge readout operation. Fig. 4 shows a CCD type solid state imaging device according to an embodiment of the present invention. This embodiment is a result of application of the present invention to a CCD type solid state imaging device provided with vertical charge transfer registers according to the second layer polysilicon four-phase drive system. In this embodiment, as in the case of Figs. 2A to 2C embodiment, the edge b of the boundary line of the (1 poly) vertical charge transfer electrode 43 and the (2 poly) vertical charge transfer electrode 45 on the side opposite the light receiving element 43 (i.e., on the side of the channel stopper 42) is biased toward the transfer direction with respect to the edge a of the boundary line on the side of the light receiving element 43 when viewed from above, so that the electrical field strength under the vertical charge transfer electrode 44 with respect to the same applying voltage and the vertical charge transfer efficiency can be increased. Further, as a result of the above arrangement, the direction in which the signal charges are read out from the light receiving elements 43 to the transfer channels 41 by the vertical charge transfer electrode 45 becomes aslant so that the readout operation can be performed more smoothly than otherwise, as in the case of the Figs. 2A to 2C embodiment. In addition, the present invention can also be used to provide an interline transfer system CCD image sensor whose structure is shown in Fig. 5. As shown, This image sensor comprises rows of vertically arranged photodiodes 52 forming light receiving portions and rows of horizontally arranged vertical CCD registers 54 both of which elements 52 and 54 are arranged horizontally in an alternative fashion. The area enclosed by the broken line designates a unit pixel 51 and photoelectric conversion is performed by a photodiode 52 in the region thereby storing signal charges. That is, signal charges generate from each photodiode 52 due to the emission of light so as to be stored. Then the stored signal charges are read out on a vertical transfer portion 54 formed by a CCD group through a gate portion 53 and are transferred in sequence to a horizontal transfer portion 55 by the charge transfer operation of the vertical charge transfer portion 54. The signal charges are then transferred to an output portion during a period corresponding to each horizontal video period enabling an imaging output to appear at the signal output terminal. The device of such a system is disclosed, for example, in USP 4,518,978 and USP 4,935,794. When the present invention is applied to the above-mentioned vertical transfer portion 54, an interline transfer CCD image sensor having an improved vertical transfer efficiency and an improved signal charge readout efficiency from the light receiving elements to the transfer channels can be obtained. Although the present invention has been described with respect to its preferred embodiments, it should be noted that the present invention is not always limited thereto and various changes and modifications may be made within the scope of the invention as described in the specification and as defined by the appended claims.

Claims: A CCD type solid state imaging device comprising: a light receiving portion having a plurality of light receiving elements (43); and a charge transfer portion (41) arranged adjacent to said light receiving portion for transferring signal charges stored in the light receiving portion through a transfer channel in a direction which is aslant to a vertical first direction, said charge transfer portion (41) having a first charge transfer electrode (44) and a second charge transfer electrode (45) adjacent to the first charge transfer electrode (44), said first charge transfer electrode (44) having a first portion crossing said transfer channel in a second direction which is perpendicular to said first direction, and a first protruding portion extending over said transfer channel in said first direction and said second charge transfer electrode (45) having a second portion crossing said transfer channel in said second direction and a second protruding portion extruding over said transfer channel in a direction opposite to said first direction, wherein: a boundary line of said first protruding part of said first charge transfer electrode (44) and a boundary line of said second protruding part of said second charge transfer electrode (45) are inclined with respect to said first direction and oriented such that when viewed from above, an edge of the boundary line of the first charge transfer electrode (44), on the side opposite the light receiving element (43) is longer than an edge of the boundary line of the first charge transfer electrode (44) on the side adjacent to said light receiving element (43) and an edge (b) of the boundary line of the second charge transfer electrode (45) on the side opposite the light receiving element (43) is shorter than an edge of the boundary line of the second charge transfer electrode (45) on the side adjacent to said light receiving element (43), said first protruding part extends in said transfer direction and said second protruding part extends opposite said transfer direction, and said both boundary lines intersect respective sides of said transfer channel, wherein said first and second charge transfer electrodes (44, 45) are arranged to be driven by four-phase pulses, and a channel stopper (42) is provided along the side of said charge transfer portion (41) opposite said light receiving portion and no channel stopper is arranged between the light receiving portion and its corresponding charge transfer portion (41) having said first and second charge transfer electrodes (44, 45).

Inventors: ISHIGAMI TOMIO, IZUMI AKIO, KOBAYASHI ATSUSHI, NAITO YASUHIKO, NAKAGAWA SHINJI, TAMUGI TAKESHI, ISHIGAMI, TOMIO, IZUMI, AKIO, KOBAYASHI, ATSUSHI, NAITO, YASUHIKO, NAKAGAWA, SHINJI, TAMUGI, TAKESHI, ISHIGAMI, TOMIO, C/O SONY CORPORATION, IZUMI, AKIO, C/O SONY CORPORATION, KOBAYASHI, ATSUSHI, C/O SONY CORPORATION, NAITO, YASUHIKO, C/O SONY CORPORATION, NAKAGAWA, SHINJI, C/O SONY CORPORATION, TAMUGI, TAKESHI, C/O SONY KOKUBU SEMICONDUCT. CORP
Applicants: SONY CORP, SONY CORPORATION
Application year: 1994
Main classification: H01L27
IPC codes: H01L27